In JP11-22609A, technology (hereunder, referred to as “prior art”) is disclosed that relates to control for switching a combustion system in an internal combustion engine in which the combustion system of the internal combustion engine can be switched from stratified combustion to homogeneous combustion, or from homogeneous combustion to stratified combustion. Since an air-fuel ratio during stratified combustion is leaner than an air-fuel ratio during homogeneous combustion, switching of the air-fuel ratio accompanies switching of the combustion system. Known methods for switching an air-fuel ratio include a method that gradually changes the air-fuel ratio so that a torque level difference does not arise. However, according to the aforementioned known method, although a torque level difference is lessened, there is the problem that the desired torque cannot be obtained, and because an air-fuel ratio that is not originally intended is used, there is a deterioration in the emissions. The aforementioned prior art has been proposed as a solution to these problems.
According to the aforementioned prior art, at a time of switching from homogeneous combustion to stratified combustion, only the target air amount is switched in a step manner before switching the target equivalence ratio in a step manner. More specifically, only the target air amount is increased in a step manner to increase the air amount in advance, and a target equivalence ratio is decreased in a step manner at a timing at which the actual air amount reaches the target air amount. That is, during a period in which the air amount is increasing with a delay relative to the target air amount, the target equivalence ratio is maintained at the ratio that is used prior to switching of the combustion system. However, when the fuel amount is decided based on the target equivalence ratio prior to switching of the combustion system, the fuel amount is in excess of an amount necessary to maintain the torque at a constant value. Therefore, in the aforementioned prior art, an increase in the torque prior to switching of the combustion system is avoided by balancing the excess of the fuel amount with the retardation of the ignition timing.
However, retardation of the ignition timing is accompanied by the possibility of misfiring. Misfiring leads to a deterioration in the drivability and a deterioration in exhaust performance. Although misfiring can be prevented by setting restrictions for retardation of the ignition timing, if retardation of the ignition timing is simply restricted, an increase in torque that is caused by an excessive fuel amount cannot be avoided. Further, in a case where the internal combustion engine that is a control object is an internal combustion engine with a turbocharger, retardation of the ignition timing causes an increase in the energy of exhaust gas, raises the turbine speed and serves to quickly raise the intake air amount. Consequently, in a case where retardation of the ignition timing is simply restricted, it takes time for the air amount to reach the target air amount due to the influence of turbo lag, and there is a concern that it will not be possible to promptly switch the air-fuel ratio.